The present invention relates generally to nondestructive testing of welds and, more particularly, to an apparatus and method using electromagnetic acoustic transducers for generating and directing horizontal waves in welded steel sheet in the direction of a butt weld, and for detecting reflected waves from weld imperfections without interference from surface irregularities and external objects.
Weld breakage lengthy coils fabricated from steel sheet results in a significant loss in production output, with a resulting loss in time and money to the steel industry. Typical industry breakage rates are between 0.5 and 1% of all welds. Generally, these welds break during the cold reduction process where the coils are cold rolled to a product having a desired gauge.
A common mill operation is where the tail of one coil is flash but t-welded to the head of an other in order that sufficient length for efficient processing and/or for a particular application is achieved. The completed weld is then clamped under a flash trimmer which is used to trim weld debris so that the sheet appears continuous. The process line halted for the welding and trimming processes, after which the larger coil is further processed. When in motion, the steel sheet travels at about 30 mph until the end of the coil approaches the welding apparatus, and another coil is welded on. There are numerous nondestructive testing techniques available to the steel industry to characterize welds. For example, measurements employing eddy currents, magnetic flux, magnetic resonance, and traditional ultrasonic methods are adequate under certain circumstances, but all of these techniques have limited applicability under constraints found in steel mills. That is, inspection procedures must not impact the normal operation of a mill's production line.
Piezoelectric-driven ultrasonic transducers have been utilized for weld inspection. See, for example, U.S. Pat. No. 3,868,847, for "System And Apparatus For Inspecting Elongated Welds," which issued to Walter A. Gunkel on Mar. 4, 1975, for a discussion of the use of these devices for weld investigation. The apparatus described therein, however, would not be useful for the high speed buttwelding applications in steel mills, since water is required to provide acoustic fluid coupling and a bearing surface for each of the transducers employed. Moreover, the apparatus must be accurately positioned over the weld bead in order to practice the Gunkel invention. This requires the additional step of indexing the steel sheet such that the apparatus might be attached. U.S. Pat. No. 4,395,913 for "Broadband Electromagnetic Acoustic Transducers," which issued to William E. Peterson on Aug. 2, 1983 more generally discusses the problems With contact procedures.
Eddy current methods are restricted to a thin skin depth of the material under investigation, and similarly to magnetic flux and resonance techniques are more suitable for crack detection in surfaces than for investigation of weld characteristics. U.S. Pat. No. 4,644,272, for "Hot Annealed Weld Inspection," which issued to Wilbert J. Janos on Feb. 17, 1987 discusses an eddy current system for hot annealed weld inspection.
The use of the Shear Horizontal (SH) mode in ultrasonic waves for inspecting weld defects is known. That is, the use of such waves is preferred for characterizing defects inside symmetric discontinuities, since many other wave modes are reflected by the weld itself, thereby generating significant background noise. See, for example, U.S. Pat. No. 4,289,030, for "Nondestructive Testing Utilizing Horizontally Polarized Shear Waves", which issued to George A. Alers et al. on Sep. 15, 1981. Moreover, in order for the welding flash to be removed, the sheet steel must be clamped in place. Since the flash knife clamps hold the steel sheet with several thousand psi of pressure, principally the SH ultrasonic waves can pass under the knife, reflect from the weld and return to a detector without being impacted by the presence of the clamps.
Electromagnetic Acoustical Transducers (EMATs) can generate SH waves in steel, do not require direct contact with the workpiece, and can be pulsed at rates which permit rapid scanning of welds. Unlike piezoelectric ultrasonic techniques and eddy current technology, EMATs require little tuning once initial setup is completed. Therefore, EMATs provide an adequate solution to the mill tradeoff between time required for weld inspection and lost production time. The design of EMAT transducers for various purposes including weld inspection is well documented. See, for example, "The Design And Use 0f Electromagnetic Acoustic Wave Transducers (EMATs)" by B. W. Maxfield and C. M. Fortunko, Materials Evaluation, 41, 1399 (1983), "Evaluating EMAT Designs For Selected Applications," by B. W. Maxfield, A. Kuramoto, and J. K. Hulbert, Materials Evaluation 45, 1166 (1987), "EMAT Designs For Special Applications", G. A. Alers and L. R. Burns, Materials Evaluation 45, 1184 (1987), and "Ultrasonic Nondestructive Evaluations Of Butt Welds Using Electromagnetic-Acoustic Transducers," by C. M. Fortunko and R. E. Schramm, Welding Journal, pages 39-46 (February, 1982). What generally separates EMAT technologies is 1. the orientation of the EMAT wires relative to the magnetic field, 2. the plane of the EMAT relative to the plane of the magnetic flux lines, and 3. the type of magnet used.
Parallel field SH wave EMAT technology is described in U.S. Pat. No. 4,295,214, for "Ultrasonic Shear Wave Transducer," which issued to Robert B. Thompson on Oct. 13, 1981, and in U.S. Pat. No. 4,100,809, for "Method For Excitation And Reception Of Ultrasonic Plate Waves In Workpieces And Devices For Realizing Same," which issued to Vladimir Timofeevich Bobrov et al. on Jul. 18, 1978. Because of the EMAT design employed, large magnet fields (1200 to 1500 Oersteds) must be generated in the sheet steel. Bobrov et al. discloses EMAT utilization at the "Magic Angle;" that is, where the EMAT generates Horizontal Shear waves in the material of interest in a direction neither perpendicular nor parallel to the direction of the magnetizing field generated by a magnet parallel to the surface of the material for providing increase coupling between the material of interest and the high-frequency electromagnetic field (See, for example, Ultrasonic Testing Of Materials, by J. Kraut Kramer and H. Kraut Kramer, 4th Ed. Springer Verlag (1990).). Bobrov et al. employ EMATs to investigate welds. Longitudinal and transverse (SH) plate waves, generated by the EMAT same and subsequently reflected from flaws in the material, are time analyzed in order to provide information concerning the nature of the flaw. In Column 20, lines 44-49, the statement is made that when both excitation frequencies are simultaneously introduced into the material, the longitudinal and transverse plate waves reflected from the flaw cannot be separated by time gating. This statement is important for analyzing butt welds, since EMAT use at the "Magic Angle" generates more than one plate mode. Although one may optimize the amount of one mode over the others depending on the angle employed, reflected signals from other sources, such as surface irregularities or the presence of trim knife clamps as described above, will interfere with signals produced by flaws in the weld. It should be mentioned that the "Magic Angle" is employed since the coupling between the EMAT and a ferromagnetic material is substantially improved away from 90.degree., even though additional modes are generated. Moreover, in lines 50-54 of Column 20, Bobrov et al. conclude that for investigation of welded seams, only focused longitudinal plate waves should be employed.
U.S. Pat. No. 4,295,214, supra, discusses the use of electromagnetic acoustic transducers for generating horizontal shear waves for the purpose of weld inspection; the times of arrival of the generated and reflected waves being correlated to determine the circumferential position of the flaw. This patent, also describes the use of a static magnetic field parallel to the surface of the material under investigation, and perpendicular to the generated wave.
Accordingly, an object of the present invention is to provide an apparatus for detecting the presence of flaws in butt welds.
Another object of the invention is to provide an apparatus for detecting the presence of flaws in butt welds in steel sheet in the presence of trim knife clamps, with negligible impact on steel mill production.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.